Telephone system



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A 7'7'ORNEV Patented Dec. 22, 1942 UNITE STATS TENT ()FFICE TELEPHONE SYSTEM Application April 23, 1941, Serial No. 389,835 Claims. (01. 179-27) This invention relates to signaling systems and particularly to telephone systems in which voice frequency signaling currents are employed.

Objects of the invention are the provision of more reliable signal transmitting and receiving means in systems employing voice frequency signaling and the prevention of false operation in response to voice currents or other interfering currents.

This invention is a signaling system comprising means for transmitting signals comprising current of a first frequency followed by current of a second frequency or for transmitting signals comprising current of the second frequency followed by current of the first frequency, and means for responding to such signals Without responding to voice currents or other interfering currents. Similar transmitting and receiving means are connected to both ends of a toll telephone line, different frequencies being used for signaling in each direction thereover. The signal transmitting means is arranged to convert direct current signals to voice frequency signals of the desired character and the signal receiving means is arranged to convert the incoming voice frequency signals to direct current signals. Means are provided for opening the talking connection over the associated toll telephone line whenever signals are being transmitted or received thereover, the connection opening means being non-responsive to voice currents or other interfering currents having a signalin frequency component.

A feature is the provision of means in a signal receiver of the above-described character for 2 holding open the talking connection between a calling line and the toll line with which the receiver is associated while line-busy 'or alltrunks-busy flashing signals are being received over the toll line.

In systems comprising tandem switching ofiices for interconnecting toll lines, signals incoming to the tandem oflice are effective to open the talking connection between the two lines and repeat the signals. Part of the signal incoming over the one line is transmitted over the second line before-the talking connection is opened; but the signal receiver at the distant end of the second line should not respond to this mutilated signal. According to a further feature of the invention, means are provided for delaying a signal response until current of the second of the two signaling frequencies has continued for a predeterminal interval of time, thereby preventing response at one end of a toll line to voice frequency signaling currents passed at a tandem switching point prior to the opening of the talking connection at the tandem point between two interconnected toll telephone lines. 1

A clear and complete understanding of the invention will be facilitated by considering a system embodying the invention and its features, one such system being represented schematically inthe drawings which forms a part of this specification. The invention'is not limited in its application to the particular system and circuit arrangements shown in the drawings but is applicable generally to any voice frequency signaling system.

Referring to the drawings:

Figs. 1 to 5, inclusive, represent a first toll ofilce comprising an operators position, an outgoing trunk circuit O-TI an incoming trunk circuit ITI, a toll line circuit TLCI a signal transmitting circuit STI and a signal receiving circuit SRI;

Figs. 6 to 10, inclusive, and 16 to 19, inclusive, represent a tandem toll office comprising toll line circuits TLC2 and TLC4, signal transmitting circuits ST2 and STA, signal receiver circuits SR2 and SR4, two-way trunk circuits TWTZ and TWT4, outgoing trunk circuits 0T2 and 0T4, toll route selectors T82 and T84, and an incoming trunk circuit 1T2;

Figs. 11 to 15, inclusive, represent a third toll office similar to that represented by Figs. 1 to 5, inclusive; and

Fig. 20 shows the relative position of Figs. 1 to 19, inclusive, to form an operative arrangement.

Reference may be had to the patent to R. E. King et al., No. 2,209,777, granted July 30, 1940, for a detailed disclosure of the cord and operators position circuits provided in each of the three toll ofiices for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The outgoing trunk circuits OTI 0T2, 0T3 and GT4, incoming trunk circuits ITI, IT2 and 1T3, the two-way trunk circuits TWTI, TWT2, TWT3 and TWT4, and. the toll route selector circuits TSZ and T54 are all similar to corresponding circuits in the aforementioned King et al. patent. The toll line circuits TLCI, TLC2, TLC3 and T104 and the associated signal transmitting and receiving circuits STI, STZ, 8T3, 3T4, SR1, SR2, SR3 and SR4 are arranged for voice frequency signaling over the associated toll lines; whereas in the aforementioned King et al. patent, composite signaling circuits are associated with the toll lines.

The toll line circuit TLC! comprises a repeat- 

